The damaging effects of sunlight on skin are well documented. Contrary to what most people believe, it is not necessary that one sunbathe to suffer the ill-effects of excessive UV exposure. In fact, significant damage can be done just by routine day to day activities in sunlight. The major short term hazard of prolonged exposure to sunlight is erythema, i.e. sunburn. In addition to the short term hazard are long term hazards such as malignant changes in the skin surface. Numerous epideminologic studies demonstrate a strong relationship between sunlight exposure and human skin cancer. Another long term hazard of ultraviolet radiation is premature aging of the skin. This condition is characterized by wrinkling and pigment changes of the skin, along with other physical changes such as cracking, telangiectasis, solar dermatoses, ecchymoses, and loss of elasticity. The adverse effects associated with exposure to UV radiation are more fully discussed in DeSimone, "Sunscreen and Suntan Products," Handbook of Nonprescription Drugs, 7th Ed., Chapter 26, pp. 499-511 (American Pharmaceutical Association, Washington, D.C.; 1982); Grove and Forbes, "A Method for Evaluating the Photoprotection Action of Sunscreen Agents Against UV-A Radiation," International Journal of Cosmetic Science, 4, pp. 15-24 (1982); and U.S. Pat. No. 4,387,089, DePolo, issued Jun. 7, 1983; all of these references being incorporated by reference herein in their entirety. Hence, although the immediate effects of ultraviolet radiation may be cosmetically and socially gratifying, the long term hazards are cumulative and potentially serious.
The fact that these effects are taken seriously by the general public is suggested by considering the sun protection product market. This market has grown considerably in recent years and many new products are introduced each year. What used to be looked upon as a seasonal business is no longer seen as such. Sunscreen agents are now included in a diversity of personal care products, particularly cosmetic type products which are worn on a daily basis.
Many consumers also desire a skin lightening benefit in combination with photoprotective benefits. Skin lightening results in a more even skin appearance by an overall lightening of basal skin tone and hyperpigmented lesions. Thus, a composition providing a combination of photoprotective benefits and skin lightening benefits is highly desirable.
Sunscreen formulas are generally based on oil-in-water and water-in-oil emulsion systems. However, many conventional systems suffer from disadvantages such as inefficient UV protection, chemical and physical instability, and unattractive aesthetic properties when applied to the skin. It has surprisingly been found that the photoprotective compositions of the present invention overcome these disadvantages. These compositions comprise a sunscreen active, a stable, hydrophobic, structuring agent, a hydrophilic surfactant, a thickening agent, and water. Without being limited by theory, it is believed that these compositions contain gel network structures, liquid crystal structures, or both. It is believed that these gel networks and liquid crystals tend to bind the available water in the composition, thereby rendering the water less available for contributing to the instability and decomposition of the active ingredients. It is also found that these compositions are well-suited for formulating with highly polar materials, e.g., inorganic sunscreen agents such as titanium dioxide, zinc oxide, and iron oxide, which are often used to enhance or boost the UV protection effects of organic type sunscreen agents. Additionally, it is found that these compositions have good aesthetic, i.e. skin feel properties. Finally, it is found that these compositions can also be easily formulated with skin lightening agents.
Therefore, it is an object of the present invention to provide novel compositions for providing protection from the harmful effects of UV radiation to the skin.
It is a further object of the present invention to provide photoprotective compositions comprising water, and having gel networks, liquid crystals or both.
It is a further object of the present invention to provide photoprotective compositions which are both chemically and physically stable.
It is a further objective of the present invention to provide photoprotective compositions which have an aesthetically appealing feel when applied to the skin.
It is a further objective of the present invention to provide photoprotective compositions which also provide a skin lightening benefit.
These and other objects will become readily apparent from the detailed description which follows.